Dembele rifled home a long-range strike on Sunday to help Barcelona to a 2-1 win over Sevilla in Tangier and their first piece of silverware for the season.
It was a rare moment of joy for the 21-year-old forward since his move from Borussia Dortmund last year for a fee which could rise to 147 million euros (131.2 million pounds).
A severe hamstring injury kept him out for nearly four months and he suffered another injury setback shortly after returning in January.
When he was fit the winger played tentatively, showing little of the imagination and quality that persuaded Barcelona to sign him as a replacement for Neymar.
Catalan media questioned his professionalism and rumours that he would leave Barcelona circulated even as he lifted the World Cup with France in July.
However, Barcelona coach Ernesto Valverde says he counts on Dembele and he started the forward alongside Lionel Messi and Luis Suarez against Sevilla.
In the second half, as Barcelona struggled to break down the Andalusians, Dembele took a step forward and became the game’s most dangerous player, ramming home the winner in the 78th minute.
"He is with us and we have high expectations for him. He has great quality and is here with us," Valverde said.
"He took part and he scored a goal, which I think is something to be happy about. As the game went on, he got better."
With Suarez and Messi both 31 and losing speed, if not quality, Dembele's urgency could give Barcelona a vital outlet this season.
The forward is capable of using both feet equally well, his goal lashed home with his right foot even though he says his left is stronger.
With his injuries cleared up and a place in the side there taking, the onus is on Dembele to deliver this season and he has started as he means to go on.
(Editing by Ed Osmond)
